Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities	Three months or less
Realized gain (loss) on available-for-sale securities	$ 0
Reclassification out of accumulated other comprehensive loss	0
Unrealized Loss on Securities	411,700,000	$ 638,100,000
Unrealized loss on securities, more than twelve months	43,300,000
Investments with other-than-temporary impairment	0	0
Contingent consideration, current	4,431,000	3,584,000
Contingent consideration, non current	5,248,000	5,082,000
Pregenen
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	134,000,000
Pregenen | Preclinical Milestones [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	14,000,000
Pregenen | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	20,100,000
Pregenen | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	99,900,000
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate unrealized loss for securities in an unrealized loss position for more than twelve months	$ 100,000	$ 100,000
Milestone achievement period	2026
Milestone discount rates	13.40%
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2016
Milestone discount rates	9.70%